I. Field of the Invention
This invention relates to a process of producing aluminum alloy sheet products having properties suitable for use in fabricating automotive parts. More particularly, the invention relates to the production of aluminum alloy sheet products suitable for fabricating automotive parts that are visible in the finished vehicles, such as automotive skin panels and the like.
II. Description of the Prior Art
The automotive industry, in order to reduce the weight of automobiles, has increasingly substituted aluminum alloy panels for steel panels. Lighter weight panels, of course, help to reduce automobile weight, which reduces fuel consumption, but the introduction of aluminum alloy panels creates its own set of needs. To be useful in automobile applications, an aluminum alloy sheet product must possess good forming characteristics in the as-received (by the auto manufacturer) T4 temper condition, so that it may be bent or shaped as desired without cracking, tearing or wrinkling. At the same time, the alloy panels, after painting and baking, must have sufficient strength to resist dents and withstand other impacts.
Several aluminum alloys of the AA (Aluminum Association) 2000 and 6000 series are usually considered for automotive panel applications. The AA6000 series alloys contain magnesium and silicon, both with and without copper but, depending upon the Cu content, may be classified as AA2000 series alloys. These alloys are formable in the T4 temper condition and become stronger after painting and baking (steps usually carried out on formed automotive parts by vehicle manufacturers). Good increases in strength after painting and baking are highly desirable so that thinner and therefore lighter panels may be employed.
To facilitate understanding, a brief explanation of the terminology used to describe alloy tempers may be in order at this stage. The temper referred to as T4 is well known (see, for example, Aluminum Standards and Data (1984), page 11, published by The Aluminum Association) and refers to alloy produced in the conventional manner, i.e. without intermediate batch annealing and pre-aging. This is the temper in which automotive sheet panels are normally delivered to parts manufacturers for forming into skin panels and the like. T8 temper designates an alloy that has been solution heat-treated, cold worked and then artificially aged. Artificial aging involves holding the alloy at elevated temperature(s) over a period of time. T8X temper refers to a T8 temper material that has been deformed in tension by 2% followed by a 30 minute treatment at 177.degree. C. to represent the forming plus paint baking treatment typically experienced by formed automotive panels. An alloy that has only been solution heat-treated and artificially aged to peak strength is said to be in the T6 temper, whereas if the aging has taken place naturally under room temperature conditions, the alloy is said to be in the T4 temper, as indicated above. Material that has undergone an intermediate batch annealing, but no pre-aging, is said to have a T4A temper. Material that has undergone pre-aging but not intermediate batch annealing is said to have a T4P temper, and material that has undergone both intermediate annealing and pre-aging is said to have a T4PA temper.
In prior U.S. Pat. No. 5,616,189, issued on Apr. 1, 1997 to Jin et al., assigned to the same assignee as the present application (and also in equivalent PCT publication WO 96/03531 published on Feb. 8, 1996), a process of producing aluminum sheet of the 6000 series is described having T4 and T8X tempers that are desirable for the production of automotive parts. The process involves subjecting a sheet product, after cold rolling, to a solutionizing treatment (heating to 500 to 570.degree. C.) followed by a quenching or cooling process involving carefully controlled cooling steps to bring about a degree of "pre-aging." This procedure results in the formation of fine stable precipitate clusters that promote a fine, well dispersed precipitate structure during the paint/bake procedure to which automotive panels are subjected, and consequently a relatively high T8X temper.
Unfortunately, sheet products produced in this way from direct chill (DC) cast ingots often suffer from a phenomenon known as roping, ridging or "paint brush" line formation (the term "roping" is used henceforth), i.e. the formation of narrow bands having a different crystallographic structure than the remaining metal resulting from the metal rolling operation and generally aligned in the direction of rolling. During subsequent transverse straining of the sheet products as they are being formed into automotive parts, these bands manifest themselves as visible surface undulations, which detract from the final surface finish of the automotive product.
Roping has been encountered by others in this art, and it has been found that roping may be inhibited by modifying the sheet production method so that recrystallisation occurs at an intermediate stage of processing. The inhibition of roping is addressed, for example, in U.S. Pat. No. 5,480,498 issued on Jan. 2, 1996 to Armand J. Beaudoin, et al., assigned to Reynolds Metals Company, and also in U.S. Pat. No. 4,897,124 issued on Jan. 30, 1990 to Matsuo et al., assigned to Sky Aluminum Co., Ltd. In these patents, roping is controlled by introducing a batch annealing step (e.g. heating at a temperature within the range of 316 to 538.degree. C.) at an intermediate stage of the sheet product formation, e.g. after hot rolling but before cold rolling, or after an early stage of cold rolling.
However, it has been found that, if an intermediate batch anneal of this kind is carried out on sheet made of 6000 series aluminum alloy, there is a reduction not only of the T4 temper, but also of the T8X temper when the alloy is subjected to the solutionizing treatment/controlled cooling steps of our prior patent application. Therefore, attempts to control or prevent roping reduce or eliminate the benefits of the favourable T4/T8X temper characteristics that are otherwise achievable for these types of alloys.
There is consequently a need for an improved process of producing aluminum automotive alloy sheet products that exhibit little or no roping while maintaining desirable T4/T8X characteristics.